1. Field of the Invention
The present invention relates to a memory device including a semiconductor device and to a semiconductor integrated circuit, such as a signal processing circuit, including the memory device.
2. Description of the Related Art
Until now, a transistor including amorphous silicon, polysilicon, or microcrystalline silicon has been used for a display device such as a liquid crystal display. A technique in which such a transistor is utilized for a semiconductor integrated circuit has been proposed (e.g., see Patent Document 1).
In recent years, a metal oxide having semiconductor characteristics, which is called an oxide semiconductor, has attracted attention as a novel semiconductor material having high mobility as in the case of polysilicon or microcrystalline silicon and having uniform element characteristics as in the case of amorphous silicon.
A metal oxide is used for various applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of the metal oxide having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (see Patent Documents 2 to 4).